This invention relates generally to an apparatus and method for spatially and temporally modulating a light beam and imaging the modulated light onto a photosensitive media.
Photographic images are traditionally printed onto photographic paper using conventional film based optical printers. The photographic industry is converting to digital imaging. One step in the digital imaging process uses images obtained from digital cameras or scanned film exposed in traditional photographic cameras to create digital image files that are then printed onto photographic paper.
The growth of the digital printing industry has led to multiple approaches to digital printing. One of the early methods used for digital printing was cathode ray tube (CRT) based printers such as the Centronics CRT recorder. This technology has several limitations related to the phosphor and the electron beam. The resolution of this technology is inadequate when printing a large format images, such as 8 inch by 10 inch photographic print. CRT printers also tend to be expensive, which is a severe short coming in a cost sensitive market. An additional limitation is that CRT printers do not provide sufficient red exposure to the media when operating at frame rates above 10,000 prints per hour.
Another commonly used approach to digital printing is the laser based engine as shown in U.S. Pat. No. 4,728,965. Such systems are generally polygon flying spot systems which use red, green, and blue lasers. Unfortunately, as with CRT printers, the laser based systems tend to be expensive, since the cost of blue and green lasers remains quite high. Additionally, the currently available lasers are not compact. Another problem with laser based printing systems is that the photographic paper used for traditional photography is not suitable for a color laser printer due to reciprocity failure. High intensity reciprocity failure is a phenomenon by which photographic paper is less sensitive when exposed to high light intensity for a short period. For example, flying spot laser printers expose each of the pixels for a fraction of a microsecond, whereas optical printing systems expose the paper for the duration of the whole frame time, which can be on the order of seconds. Thus, a special paper is required for laser printers.
A more contemporary approach uses a single spatial light modulator such as a Texas Instruments digital micromirror device (DMD) as shown in U.S. Pat. No. 5,061,049. Spatial light modulators provide significant advantages in cost as well as allowing longer exposure times, and have been proposed for a variety of different printing systems from line printing systems such as the printer depicted in U.S. Pat. No. 5,521,748, to area printing systems such as the system described in U.S. Pat. No. 5,652,661. One approach to printing using the Texas Instruments DMD shown in U.S. Pat. No. 5,461,411 offers advantages such as longer exposure times using light emitting diodes (LED) as a source. See U.S. Pat. No. 5,504,514. However, this technology is very specific and not widely available. As a result, DMDs are expensive and not easily scaleable to higher resolution. Also, the currently available resolution is not sufficient for all printing needs.
Another low cost solution uses LCD modulators. Several photographic printers using commonly available LCD technology are described in U.S. Pat. Nos. 5,652,661; 5,701,185; and 5,745,156. Most of these involve the se of a transmissive LCD modulator, for example U.S. Pat. Nos. 5,652,661 and 5,701,185. While such a method offer several advantages in ease of optical design for printing, there are several drawbacks to the use of conventional transmissive LCD technology. Transmissive LCD modulators generally have reduced aperture ratios and the use of Transmissive Field-Effect-Transistors (TFT) on glass technology does not promote the pixel to pixel uniformity desired in many printing applications. Furthermore, in order to provide large numbers of pixels, many high resolution transmissive LCDs possess footprints of several inches. Such a large footprint can be unwieldy when combined with a print lens. As a result, most LCD printers using transmissive technology are constrained to either low resolution or small print sizes. Also, to print high resolution 8 inch by 10 inch images with at least 300 pixels per inch requires 2400 by 3000 pixels. Transmissive LCD modulators with such resolutions are not readily available. Furthermore, each pixel must have a gray scale depth to render a continuous tone print and do so uniformly over the frame size, which is not available in this technology.
An alternate approach is to utilize reflective LCD modulators as is widely accepted in the display market. Most of the activity in reflective LCD modulators has been related to projection display. The projectors are optimized to provide maximum luminous flux to the screen with secondary emphasis placed on contrast and resolution. To achieve the goals of projection display, most optical designs use high intensity lamp light sources. Additionally, many projector designs use three reflective LCD modulators, one for each of the primary colors, such as the design shown in U.S. Pat. No. 5,743,610. Using three reflective LCD modulators are both expensive and cumbersome. For projectors using a single reflective LCD modulator, color sequential operation is required. To maintain the high luminosity in combination with the color sequential requirements, a rotating color filter wheel is sometimes employed. This is yet another large, moving part, which further complicates the system.
The recent advent of high resolution reflective LCDs with high contrast greater than 100:1, presents possibilities for printing that were previously unavailable. See U.S. Pat. Nos. 5,325,137 and 5,805,274. Specifically, a printer may be based on a reflective LCD modulator illuminated sequentially by red, green and blue, light emitting diodes. The reflective LCD modulator may be sub-apertured and dithered in two or three directions to increase resolution.
Dithering has been applied to transmissive LCD systems due to the already less than perfect fill factor. Incorporating dithering into a reflective LCD printing system would allow high resolution printing while maintaining a small footprint. Also, because of the naturally high fill factor present in many reflective LCD technologies, the dithering can be omitted with no detriment to the continuity of the printed image.
The use of a reflective LCD serves to significantly reduce the cost of the printing system. Furthermore, the use of an area reflective LCD modulator sets the exposure times at sufficient length to avoid or significantly reduce reciprocity failure. The progress in the reflective LCD device field made in response to needs of the projection display industry have provided opportunities in printing applications. Thus, a reflective LCD modulator designed for projection display can be incorporated into the printing design with little modification to the LCD itself. Also, by designing an exposure system and data path with an existing projection display device allows incorporation of an inexpensive commodity item into a print engine.
Of the reflective LCD technologies, the most suitable to this design is one which incorporates a small footprint with an integrated Complementary Metal Oxide Semiconductor (CMOS) backplane. The compact size along with the uniformity of drive offered by such a device will translate into better image quality than other LCD technologies. There has been progress in the projection display industry towards incorporating a single reflective LCD, primarily because of the lower cost and weight of single device systems. See U.S. Pat. No. 5,743,612. Of the LCD technologies, the reflective LCD with the silicon backplane can best achieve the high speeds required for color sequential operation. While this increased speed may not be as essential to printing as it is for projection display, the higher speeds can be utilized to incorporate additional gray scale and uniformity correction to printing systems.
While the reflective LCD modulator has enabled low cost digital printing on photosensitive media, the demands of high resolution printing have not been fully addressed. For many applications, such as imaging for medical applications, resolution is critical. Often, the resolution provided by a single reflection LCD modulator is insufficient. It then becomes necessary to create an image wherein multiple images are merged to create a single high-resolution image. Creating a merged image without artifacts along the borders, or in regions where image data may overlap, is desirable. While juxtaposing or spatially interweaving image data alone may have been attempted in previous applications, such a superposition of images with reflective LCDs provides images of high quality without compromising the cost or productivity of the print engine. By utilizing polarization based modulation, a print engine can utilize light already available in the optical system.
While similar methods have been employed in projection systems, the use of a reflective LCD and dual polarization has not been used in the field of printing. In particular, because of the time delay involved in printing, artifacts present in the stitched image, as well as differences between multiple modulators can be corrected. These can be compensated through software adjustments. This approach is difficult for applications such as projection display, particularly motion imaging. Additionally, in a printing system, all three colors used to create a composite image need not be simultaneously displayed as in a projection system.
Modulator printing systems can incorporate a variety of methods to achieve gray scale. Texas Instruments employs a time delayed integration system that works well with line arrays as shown in U.S. Pat. Nos. 5,721,622, and 5,461,410. While this method can provide adequate gray levels at a reasonable speed, line printing Time Delayed Integration (TDI) methods can result in registration problems and soft images. Alternate methods have been proposed particularly around transmissive LCDs such as the design presented in U.S. Pat. No. 5,754,305.
Because photosensitive media responds differently to light of different wavelengths it may be necessary to image one color for a longer period of time than another color. For example, it takes longer to image red on a photosensitive media than it takes to image blue or green.
It is desirable to reduce the time required to expose a photosensitive media to multiple colors. It is also desirable to increase the resolution of a photographic image, using available technology cost, reduce reciprocity failure, while preserving adequate gray scale.
An object of this invention to provide for a high pixel density color image at a media exposure plane in a silver halide (AgX) printing system; and to provide means by which to utilize a high site density spatial light modulator to create digital images for imaging onto photographic media.
Briefly, according to one aspect of the present invention, a method of printing to a photosensitive media comprises the steps of imaging light from a light source through an optics assembly to a polarization beamsplitter. The polarization beamsplitter produces a first polarized state and a second polarized state. The first polarized state is directed to a first filter to a first spatial light modulator. The first spatial light modulator is addressed with a first color signal to modulate the first polarized beam which is reflected back through the polarization beamsplitter. A second polarized light is passed through a second filter to a second spatial light modulator. A second color signal causes said second spatial light modulator to reflect a second modulated light beam through the polarization beamsplitter. The first and second modulated light beam are focused through print lens onto a photosensitive media. Light emitting diode sources are imaged color sequentially through a polarizer, spatial uniformizing optics and a polarizing beamsplitter to create essentially telecentric illumination at the plane of a spatial light modulator. The spatial light modulator is comprised of a plurality of modulator sites in two dimensions. Individual modulator sites rotate the polarization state of incoming light and reflected light passes again through the polarizing beamsplitter cube. Light is then imaged through a print lens assembly and an additional polarization element onto a media plane. The media is exposed in a color sequential manner with a two dimensional color image. The media is then moved to a second position and a new image is printed.
In one embodiment, the printer incorporates at least two reflective liquid crystal display devices, illuminated by multiple wavelengths, which expose photosensitive media. In the first configuration, two LCDs are placed at opposing facets of a beam splitting cube. White, composite LEDs or white light source, light illumination is divided into TE and TM polarization states by the polarizing beamsplitting cube. One facet of the cube is followed by a red filter or dichroic, the other facet contains blue and green filters switching or rotating, or blue/green dichroics. Light passing through the red filter illuminates a LCD addressed with data corresponding to the red content of an image. Light passing through the blue and green filters illuminates a LCD addressed with the blue and then green content of an image. The light from both facets passes through a single print lens and is recombined at the image plane. Because the red exposure requires a longer exposure time, the blue and green data can be flashed sequentially in the time required to illuminate in red. The media is exposed and advanced.
In a second major embodiment, an alternate light source such as a halogen lamp is used in conjunction with color filters. In a third embodiment images are printed side by side and twice the throughput is achieved.
The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below.